He also killed his wife Izabela Rzeszowska, Kinga’s pal Julia De La Haye, five, and her mum Marta De La Haye, 34.

Earlier he had stabbed dad-in-law Marek Garstka, 56. The court heard they were all killed at the family home in St Helier, Jersey.

Solicitor General Howard Sharp, prosecuting, said Kacper was “stabbed on two separate occasions with two different kitchen knives”.

His older sister ended up on the floor of the lounge, not far from her brother.

Mr Sharp said Mrs Rzeszowska had managed to flee the attack and run out into a courtyard. She climbed back in through the bathroom window and made a “desperate attempt” to call the police using her dad’s mobile.

But Mr Sharp said: “The number she dialled was 997, which is the Polish emergency services number.”

Mrs Rzeszowska then made it out into the street but was chased by her husband and stabbed again in full view of residents.

Neighbours tried to intervene and 31-year-old Rzeszowski started stabbing himself as he went back inside the flat.

He slumped to the ground with a collapsed lung.

Mrs De La Haye also tried to leave the flat but Mr Sharp added: “She staggered out into the street and collapsed in front of some scaffolding outside the flat next door.”

Mr Sharp described Rzeszowski, who worked as a builder before his arrest, as a “pressure cooker who lacks a safety valve”.

He said: “He has difficulties controlling his emotions and communicating his frustrations and worries. He has a history of violence.”

Rzeszowski pleaded guilty to manslaughter through diminished responsibility in April, but the pleas were not accepted.

But the Crown argues that he was not suffering an “abnormality of the mind”.

The Royal Court in St Helier heard Rzeszowski’s rampage took place against a backdrop of increasing marital difficulties.

Mr Sharp said 30-year-old Mrs Rzeszowska had told her husband that she had been having an affair.

Rzeszowski was remanded in Broadmoor Hospital, Berks, after his arrest. The case continues.